Insights into incipient soot formation by atomic force microscopy

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ProceedingsoftheCombustionInstitute37(2019)885–892

www.elsevier.com/locate/proci

Insights

into

incipient

soot

formation

by

atomic

force

microscopy

Fabian

Schulz

a,1

,

Mario

Commodo

b,1

,

Katharina

Kaiser

a

,

Gianluigi

De

Falco

c

,

Patrizia

Minutolo

b

,

Gerhard

Meyer

a

,

Andrea

D‘Anna

c,∗

,

Leo

Gross

a,∗

a_IBM_Research_{− Zurich,}_{Säumerstrasse}_4,₈₈₀₃_Rüschlikon,_Switzerland b_Istituto_di_Ricerche_sulla_Combustione,_CNR,_P.le_Tecchio_80,₈₀₁₂₅_Napoli,_Italy

c_Dipartimento_di_Ingegneria_Chimica,_dei_Materiali_e_della_Produzione_Industriale_-_{Università degli}_Studi_di_Napoli FedericoII,P.leTecchio80,80125Napoli,Italy

Received30November2017;accepted13June2018 Availableonline3July2018

Abstract

Combustion-generatedsootparticlescanhavesignificantimpactonclimate,environmentandhuman health.Thus,understandingtheprocessesgoverningtheformationofsootparticlesincombustionisatopic ofongoingresearch.Inthisstudy,high-resolutionatomicforcemicroscopy(AFM)wasusedfordirect imag-ingofthebuildingblocksformingtheparticlesintheearlystagesofsootformation.Incipientsootparticles werecollectedrightaftertheparticlenucleationzoneof aslightlysootingethylene/airlaminar premixed flameatatmosphericpressureandanalyzedbyAFMafterarapidsublimationprocedure.Ourdatashed lightononeof themostcomplexandstilldebatedaspectonsootformation,i.e.,thenucleationprocess. Themolecularconstituentsof theinitialparticleshavebeenindividuallyanalyzedindetailintheir chemi-cal/structuralcharacteristics.Ourdatademonstratethelargecomplexity/varietyofthearomaticcompounds whicharethebuildingblocksoftheinitialsootparticles.Nevertheless,somefundamentalandspecific char-acteristicshavebeenclearlyascertained.Theseincludeasignificantpresenceofpenta-ringsasopposedto thepurelybenzenoidaromaticcompoundsandthenoticeablepresenceofaliphaticside-chains.Inaddition, therewereindicationsforthepresenceofpersistentπ radicals.IncipientsootwasalsoinvestigatedbyRaman spectroscopy,theresultsofwhichagreedintermsofchemicalandstructuralcompositionoftheparticles withthoseobtainedbyAFM.

Keywords: Incipientsoot;Nucleation;Atomicforcemicroscopy;Atomicresolution;Ramanspectroscopy

∗_{Corresponding}_authors.

E-mailaddresses:anddanna@unina.it (A.D‘Anna), lgr@zurich.ibm.com(L.Gross).

1_These_authors_contributed_equally.

https://doi.org/10.1016/j.proci.2018.06.100

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1. Introduction

Soot formation in combustion is a topic of greatrelevancebecauseoftheadverseimplications onthehumanhealthandontheenvironment.Soot particles are known to be harmful, particularly whentheirsizeisatthenanoscale,having numer-ousandascertained negativehealtheffects[1–3]. Inaddition,theemissionofsootintheatmosphere has complex implications in affecting the envi-ronment andparticularly the climate change[4]. Therefore,theneedtoreducetheemissionofthese carbonnanoparticlesintotheatmospherepushes for a better understanding of the chemical and physical mechanisms behind the soot formation process. However, understanding soot formation in combustion is not atrivial task. Soot forma-tion/evolutionchemistry,infact,isacomplex pro-cessinvolvinggas-phasefuel-richflamechemistry, gas-to-particle transition, particle coagulation and/or coalescence,heterogeneous growth, parti-cle agglomeration, carbonization and oxidation. Amongall,itisquitecertainthatthemostcomplex, yet unresolved and scientifically intriguing phe-nomenonisthenucleation[5–8].Itiswellknown thatpolycyclicaromatichydrocarbons(PAHs)are keycompounds acting asbuilding blocks. How-ever,manyaspectsarestillunderdebateandobject of ongoing experimentaland theoretical studies. Theseinclude:size[9],chemicalcomposition,e.g., presenceofheteroatomsandinparticularof oxy-genatoms[10],presenceofpenta-rings[11],aswell as cross-linking among two or several aromatic groups[12,13]andaliphaticsidechains[14].Once formed,thesearomaticstructuresbeginto assem-bleintojust-nucleatedparticlesorclusters,whose sizeisusuallyoftheorderoffewnanometers,i.e., 2–3nm[5–8].Twodistinctclusteringprocesseshave beenhypothesizedovertheyearsandoftenreferred toasphysicalandchemicalpathways.Theformer considersthesootnucleation/clusteringprocessas ahomogeneous condensationprocesspurelydue totheoccurrenceofphysicalvanderWaals attrac-tiveforcesbetweenaromaticmolecules,whichare commonlyassumedinsootmodelstobepyreneor slightlylargerperi-condensedPAHs.Bycontrast, thelatter considers sootinception as proceeding throughtheformationofchemicalbonds connect-ingtwo ormore aromatics toform polymer-like assembliesand/ormacromolecules[5].

To date, most of the data on incipient soot molecular/chemical composition derive from mass spectrometric studies [15–18] and opti-cal/spectroscopicmethods[19–23].Unfortunately, theformerhaslimitedcapabilityinproviding de-tailedinformationormolecularassignmentasthe massofthemoleculeincreases,andthelatteryields only global chemical/structural information for suchcomplexensemblesof chemicalcompounds. Tobetterunderstandthechemicalcompositionof precursorparticles it isnecessary touse

analyti-cal methods capable of providing more detailed information on the molecular structure of the aromatic building blocks forming the incipient sootparticles.

Scanningprobemicroscopy(e.g.,scanning tun-neling microscopy, STM and atomic force mi-croscopy,AFM)isuniquelycapabletoimage ad-sorbates at the atomic scale. In particular, it is possible to image single molecules with atomic resolutionusingAFM[24].This major advance-mentisenabledbyusingatomicallyfunctionalized tips,which probe the repulsive forces above sin-glemolecules[25].Consequently,thismethodwas alsoappliedtomeasurebondorder[26],identify theatomicstructureofnaturalcompounds[27]as wellastheconstituentsofcomplexmolecular mix-tures[28,29]andfollowtheintermediatestepsof on-surfacereactions[30,31].

ComplementarytoAFM,STMgivesaccessto electronicpropertiesofadsorbates.Whenusingan ultrathininsulatinglayerassubstrate,STMcanbe used,e.g.,tomapmolecularorbitaldensities[32– 34].Thecombinationofthesetwotechniques pro-videsapowerfultooltoidentifythechemical struc-tureofunknownmolecules[28].

Inthiswork,incipientsootnanoparticles gener-atedinalaminarpremixedethylene/airflamehave beendecomposedintotheirmolecularconstituent buildingblocksandinvestigatedbyhigh-resolution AFM. These measurements yield importantand uniqueinformationonthetypesandchemical char-acteristicsofthearomaticbuildingblocksforming thejust-inceptedparticles.Inaddition,sampled in-cipientparticleshavealsobeeninvestigatedby Ra-manspectroscopy.Thefindingsobtainedfromthe wholesetofanalysisconstituteanimportantstep forwardintheunderstandingof thechemistryof thesootparticlesattheearlystageoftheir forma-tion,thuscontributingtothefoundationand guid-anceoffuturedetailedsootmodels.

2. Experimental

An atmospheric-pressure laminar premixed ethylene–airflame was stabilizedona McKenna burner havinga diameter of 6cm.The cold gas velocitywassetat9.8cm/sandcarbontooxygen (C/O) atomic ratio fixed at 0.67, i.e., equiva-lenceratio Ф=2.03. The accuratesettingof the combustionparametersensuredlong-term stabil-ity of gas-flow velocity, composition and flame temperature.

Incipientsootparticleswerecollectedfromthe flameataheightabovetheburner(HAB)of8mm. Thecombustionproductswereextractedfromthe flamecenterlinebymeansofahigh-dilution hori-zontaltubularprobe[35].Specifically,the combus-tionproductsweresampled throughaverysmall orifice, i.e., 200 μm,located on the bottom side of the probe and rapidly mixed with N2, thus

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providing a dilution ratio of 1:(3·103₎ _[36]_._This samplingprocedurepreventsparticlesfrom coagu-lating,andallowsquenchingthechemicalreactions throughoutthesamplingline[35].Flame tempera-tureprofiles,withandwithoutprobe,havebeen re-portedelsewhere[36].Theparticlesizedistribution (PSD) was measured on-line using a differential mobilityanalyzer(DMA)systemcomposedbyan electrostatic aerosol classifierVienna–typeDMA (TapCon3/150,sizerangeof 1–40nm),anX-ray diffusionchargingsource(TSIMod.3088),anda Faradaycupelectrometer. Fortheoff-line analy-ses, astainless-steelaerosolfilterholder contain-ingaquartzfilter(WhatmanQMAgrade,47mm) waspositionedon-linedownstreamofthedilution tubular probe for soot nanoparticles collection. Gastemperatureatthefilterlocationwas350K. Atthistemperatureandinthehighdilution condi-tion,thecondensationof gas-phasePAHsshould bedisfavoreduptothesizeof ovalene.The sam-plecollectionlasted14hourstogatherenough ma-terialonthefilterforoff-lineanalysis.Theflame temperaturewasmonitoredandfoundtobe con-stantwithin± 10°Cduringthesamplingtime. Pre-liminarycontrolexperimentsonthesampling pro-cedure/reproducibilitywereperformedbyRaman spectroscopy.

STMandAFMmeasurementswerecarriedout inahome-builtcombinedSTM/AFMset–up op-eratedatlowtemperature(T≈ 5K)andultra-high vacuum (UHV, p≈ 1× 10−10 mbar). The micro-scopewasequippedwithaqPlusquartzcantilever [37]operated in the frequency modulationmode [38] (resonance frequency f0≈ 28.8 kHz, spring constantk≈ 1.8kN/m,qualityfactorQ≈ 100,000

andoscillationamplitudeA≈ 0.6A).˚ Thebias volt-age was appliedto thesample andSTMimages weretakenintheconstant-currentmode.AFM im-agesweretaken intheconstant-heightmodeand at0Vbiasvoltage.Positiveheightoffsets,z, cor-respondtoa decreaseof thetip-sample distance withrespecttotheSTMset-point.ACu(111)single crystalpartially coveredwith(100)-oriented,two monolayer (ML) thickNaCl islands [denotedas NaCl(2ML)/Cu(111)] was usedas substrate. The Cu(111)crystalwascleanedbyrepeatedcyclesof Ne+ sputtering and annealing to ∼800K. Sub-sequently, NaCl was evaporated onto the clean Cu(111)surfaceat∼ 270K.Smallamountsof car-bonmonoxide(CO)weredosedontothesurface at∼10KfortipfunctionalizationbyadmittingCO into theUHVchamber. The microscopetipwas madefrom25μm-thickPtIrwiresharpenedwith afocusedionbeam.AcleanandsharpCutipwas preparedinsitubydeliberateindentationsintothe Cu(111)substrate.Thetipapexwaspassivatedby pickingupasingleCOmoleculefromthesurface [39].

Thesootnanoparticlesweredepositedontothe substrate by applying a small amount of

mate-rialfromthequartzfilterontoapieceof silicon waferbygentlypressingthefilterontothewafer. The wafer was subsequently introducedinto the UHVsystemandflash-heatedbyresistiveheating toevaporate themoleculesontothecold sample (T≈ 10K). Thereby, the wafer was heated from room temperature to ∼900K within a few sec-onds.Wehavepreviouslycarriedoutseveral con-trolexperimentsconcerningreproducibility, relia-bilityandstatisticalsignificanceofthispreparation method[29,40].PAHs includingarchipelago-type moleculeswithmassesuptoabout600Da[40] sub-limeintactfromthesiliconwafer.Thus, molecu-larmixturescontainingmoleculeswithmasses be-low 600Dawill be depositedwithout significant changesincompositionandstructureonthe sur-face.This has been corroboratedby comparison ofAFMdataonmixtureswithmassspectroscopy data[29].Forthesampleof nascentsoot investi-gatedinthisstudy,showing meanmassesonthe order of 350Da and only a very small part of molecules>600Da, the flash-heating method is suitable.

STM/AFMexperimentswereperformedwithin afewdaysaftersampling. Weareconfidentthat the sample does not undergo significant chemi-cal/structuralmodificationduringsuchtimeperiod ascorroboratedbythereproducibilityof the Ra-man andlight absorption spectraof thesample overseveraldays.Wealsodidnotobserve isomer-izationordecompositionoftheimagedmolecules duringSTM/AFMcharacterization.

Raman spectroscopy was performed by po-sitioning the filter without further manipula-tion under the microscope (Horiba XploRA) equippedwitha100Xobjective(NA0.9Olympus), a Nd:YAG laser (λ =532nm, 12mW maximum laserpoweratthesample)anda200μmpinhole for confocal photons collection. System calibra-tionwasperformedagainsttheStokesRaman sig-nalof puresiliconat520cm−1.A.Thepowerof theexcitationlaserbeamwasattenuatedto1%to avoidstructuralchangesofthesampledueto ther-maldecomposition whenusingan accumulation-exposuretimeof5cyclesof30seach.Atotalof10 spotswererandomlyselectedtoverifythe homo-geneityofthesampleandfinallyaveragedtoobtain astatisticallyrelevantRamanspectrum.The stan-darddeviationwasoftheorderoffewpercent.

3. Resultsanddiscussion

The whole setof experiments was conducted onincipient sootparticlescollected froma lami-narpremixedflame atafixed C/Oratioand dis-tance from the burner exit, i.e., C/O=0.67 and HAB=8mm. In this flame condition, particles presentedaunimodalPSDwithamaximum posi-tionedbetween2and2.5nm(mobilitydiameter), asshowninFig.1.

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Fig.1.Particlesizedistributionat8mmabovetheburner ofthelaminarpremixedflamemeasuredbyDMA.

Thisparticlemodeiswellrepresentativeofthe onsetofsootformationasalsodescribedina pre-viouswork[36].Onlyathigherresidencetimes,i.e., atincreasedHABs,theparticlegrowthprocessdue tocoagulation/coalescenceandsurfacemass addi-tionwas evidencedfromthe formationof a sec-ondmodecenteredatlargerdiametersinthePSD, whichclearlyappearedasbimodal[36].

The particles collected on the quartz filter weresubsequentlyinvestigatedbylow-temperature atomic-resolution AFM/STM, andRaman spec-troscopy.

A typical STM overview image of the NaCl(2ML)/Cu(111) substrate after sublima-tion of soot nanoparticles is shown in the left panelof Fig.2.Located onthebottompartisa NaCl(2ML)island,andseveralCOmoleculesand sublimatedsootparticlesareadsorbedonthe sur-face.Importantly,duetotheparticledepositionvia

flash–heating, any clusterspotentially formed by

vanderWaalsstackingduringthenucleation pro-cessareexpectedtobreakdownintoitsaromatic buildingblocks.TherightpanelofFig.2showsan STMimageandtwohigh-resolutionAFMimages taken at different tip-sample distances for three characteristiccompoundsfoundinthisstudy(M1, M2, M3), respectively. The AFM images show thecarbonskeletonofthePAHcoresasrepulsive (bright)contrast,whichbecomesmorepronounced asthetip-sampledistanceisdecreased.Inaddition, thebondsgetslightlydistortedatcloserdistances, which is due to the flexibility of the CO at the tip apex [26,41,42]. Comparison with previous AFMstudies of referencemoleculesalso enables the assignment of methyl groups [28], aliphatic moieties [40] and oxygen-containing side groups such as ketones [43] and methoxy groups [27]. The proposed chemical structures of M1, M2, andM3 are shown next to the AFM images in Fig.2.Specifically,thecompoundM1(C24H12)is made of a central peri-condensed aromatic core composed by six fused benzene rings and one additionalfive-memberedringontheperipheryof themolecule.

Compounds M2 (C31H16) and M3 (C44H20) showalargerconjugationlength,i.e.,ahigher num-ber of fused benzene rings,containing 9and 13 benzenerings,respectively.M3alsocontainstwo penta-rings,similar toM1.Indeed,ourstudy re-vealsthatsuchperipheralfive-memberedringsare acommonmotif inthemolecularconstituentsof incipientsootnanoparticles.However,incontrast to M1,the two penta-ringsin M3 are not fully aromatic. Their two outer carbon atoms are sp3

hybridized anddoublyhydrogenated, thus form-ingcycloaliphaticmoieties [40].This causestheir brightcontrast inthe AFMimages andthe dis-tortionsatclosetip-sampledistance.Interestingly,

Fig.2. Leftpanel:STMoverviewimage(I=0.5pA,V=0.4V)ofthesampleafterdepositionofsootparticles.Scalebar is10nm.Rightpanel:STMandAFMimagesofmoleculesM1[onNaCl(2ML)/Cu(111)],M2[onCu(111)]andM3[on NaCl(2ML)/Cu(111)]atdifferentset-pointszfrom(I=0.5pA,V=0.2V).Scalebarsare5A.˚

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Fig.3.AFMimagesandchemicalstructureofsomerepresentativemolecules,aswellasmolecularorbitaldensities mea-suredbySTMforM10andcorrespondingDFTsimulations.“R” labelsdenoteunidentifiedpartsofthemolecules.Scale barsare5A.˚

anotheraliphaticmoietycanbeidentifiedin com-poundM2,whichcontainsasidegroupattributed toamethyl(–CH3).Thepresenceofaliphatic con-tributionsin just-nucleatedsootparticleswas re-portedinpreviousstudies[18–20,23].However,to thebestof ourknowledge,ourdatarepresentthe firstdirectobservationofaliphatic-substituted aro-maticsinsootparticles.

The aim of this workis tocharacterize com-monmolecularstructuresandmotivesinearlysoot formationbyatomic-resolutionAFM.However,a statisticalanalysisandquantificationof the com-poundsgoesbeyondthescopeofthiswork.A com-parisonwithmassspectrometryindicatedthe sta-tisticalsignificanceof thesamplingbyAFM[29]. However, quantification in complex mixtures by AFMremainsextremelychallengingbecauseofthe largenumberofmoleculesthatneedtobeimaged andbecauseallmoleculesinsamplingareasshould be assigned.Here, wefocus ouranalysis on pla-narPAHsandaliphaticchains,whichpermit reli-ablechemicalstructureassignmentbyAFM.Such moleculesmade upabouthalf of theadsorbates foundonthesamplesurfaceafterpreparation.The other half of themoleculeswere toonon-planar and/orverymobileundertheinfluenceofthetip and for that reasons could not be imaged with atomicresolution.

Adetaileddiscussionof theentiresetof mea-suredmoleculesandtheirassignedstructurewould be out of scope of this article andwill be pro-videdinaseparatemanuscript.Instead,wereport in Fig.3further selected structureswhich repre-sent the different groups of molecules observed

during the STM/AFM measurements. Molecule

M4(C28H13),forexample, iscomposedof seven fused benzene rings, i.e., coronene, and two ad-ditionalfive–memberedringsontheperipheryof themolecule.Becauseoneofthepenta-rings con-tains a doubly-hydrogenated carbon, this com-poundrepresents oneof thefewcaseswherethe assignedstructuresuggeststhepresenceofan aro-matic radical.However, theassignmentof thein part aliphatic penta-rings is tentative and needs tobecorroboratedfurtherbythemeasurementof knownstandards.Theroleofaromaticπ radicals, andparticularlythecontributionofradicalsitesas activesitesforcarbonadditionhaslongbeing hy-pothesizedinthesootformationprocess[44].In ad-dition,therelevanceofaromaticradicalsresulting fromlocalizedπ electronintheparticlenucleation process andduring the subsequent mass growth has been recently remarked in Wang’s review paper[6].

MoleculeM5(C24H12)containsfivefused ben-zeneringsandtwopenta-ringsandisoneof the smaller PAH cores found in our study. Interest-ingly,itsshapeismoreelongatedandshows par-tial cata-condensation, while most of the identi-fiedstructureswere purelyperi-condensed. Com-pound M6 represents one of the most complex structuresfoundduringthemeasurements.It con-tainsaPAHcoremadefromsevenbenzenerings andtwopenta-rings.Incontrasttothepreviously discussedstructures, herethepenta-ringsarenot locatedatperipheralzigzagregionsbutembedded inarmchairedges.Inaddition,theoutercarbonof bothpenta-ringsisdoublyhydrogenated.Finally,

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oneof thebenzeneringsiscata-condensedatthe peripheryandfurtherconnectedtoalong,aliphatic sidegroup.Thischaincanbeidentifiedasanalkyl chain[(CH2)n],basedontheverybrightcontrastin theAFMimageanditscharacteristicmodulation [40].

Compounds with low conjugation length to-getherwithsomepurelyaliphatic oneswerealso observed,e.g.,M7,M8,andM9.Thesemolecules included:substitutedbenzenemolecules,e.g., com-poundM7;aliphatic-linkedaromaticcompounds, e.g.,M8;andsomeisolatedaliphaticchains,e.g.,

M9.InM8,bothsix-memberedringsaretiltedout of thesurfaceplane,indicatingstericalhindrance duetotheattachedsidegroups.

On the other hand, M10 (C52H26; 650.76u;

H/C=0.5) is one of the largest identified com-pounds. Interestingly, although the majority of imagedsootfragmentswere intheformof

“sin-glet” isolatedPAHs,thestructureofM10suggests the covalent linking of two aromatic “islands”. To confirm the assigned molecular structure of

M10, its highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbital densities weremeasuredbySTM, theresultsbeingshown inFig. 3alongwithM10.These werecompared toabinitioelectronicstructurecalculationsusing the assigned chemical structure as input. For the calculations,we employed density functional theory(DFT)withthePBEexchange-correlation functional[45],asimplemented in theFHI-aims code[46].TheHOMOandLUMOcalculatedfrom DFT are shown in Fig. 3next to the measured orbitaldensities.Theagreementisexcellent,thus confirming our assigned structure. Importantly,

M10 represents the first experimental evidence of so-called “archipelago” compounds within the pool of molecules participating in the soot formation.

Itisworthnoticingthatmostof theobserved penta-rings are positioned mainly on the zigzag regionof thehexagonal aromatic structure.This isconsistentwith theirformationby additionof oneacetylenemolecule andsubsequentring clo-sure[47].Bycontrast,bowl-shapedPAHs contain-ingfive-memberedring surroundedbyhexagons, suchascorannulene(C20H10)orsimilararomatics, wererarelyobserved.

In general, we observed that the majority of the carbon atoms on the periphery of the aro-maticmolecules wereeitherin“zigzag” or

“arm-chair” conformation,while“cove” areveryrareand “fjord” types[48]wereneverobserved.

Recentliteraturealsoreportsacontributionof oxygenated compoundsintheformation of soot particlesbothindiffusion[18]andpremixedflame conditions[9,49].Inthecurrentstudy,noneofthe moleculesidentifiedbyAFMpresentedclear evi-denceofoxygensincorporatedintothePAHcore orasketones.However,oxygencouldbecontained

Fig.4. Ramanspectrumofincipientsootnanoparticles.

in non-planar fragments and bulky side groups, whicharechallengingtocharacterizewithAFM.

Just-nucleatedsootparticles werealso investi-gatedbyRamanspectroscopy.Thisspectroscopic technique is known to be an excellent tool for collecting detailed information about the elec-tronicandchemicalstructureofanycarbon-based material, including amorphous carbon [50]. In recent years, Raman spectroscopy has also be-come acommon analytical methodforsoot mi-cro/nanostructureinvestigations[51–53].

The first-order Raman spectrum, 1000– 1800cm−1,ofthesampleisreportedinFig.4.It presentstwomainfeatures,i.e.,theGandDpeak, whichnormallydominateintheRamanspectrum of any disordered carbonaceous materials [50]. TheabilityofRamanspectroscopytoprobe chem-ical/structural information of carbon materials liesinthefactthatthepresenceof defectsinthe

sp2_aromatic_network_allows_the_activation_of _the

Raman D mode at ∼1350 cm−1, prohibited in theperfecthexagonallattice.Edgeeffectsinsmall graphitic domains are one major source of this band. Conversely, the G band, at ∼1600 cm−1, dueto every sp2 _carbon,_is _mostly _insensitive_to

defectsandonlypresentssmall changesinwidth andpositionof themaximumasfunctionof the differentcarbonstructures[50].Besidesthesetwo bands,other interestingfeaturescan beobserved inthespectrumofFig.4.Theseincludearelevant signalin thevalleyregion between theDandG band and some defined bumps in the spectral region 1000–1250 cm−1. For disordered carbon materialsandsoot,variousassignmentshavebeen proposedforsuchfeatures[50,51,54].Ferrariand Robertson[50]attributed thepeak at1150 cm−1 totrans-polyacetylene. The presenceof aliphatic chains inincipient sootareclearly demonstrated bytheAFMresultsreportedhere,e.g.,soot frag-mentM9ortheside-groupin M6.However,the observedaliphaticchainsaremainlyalkylchains,

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i.e.,polyethylene, whichalsopresent vibrationin thisspectralrange[55].Suchchains,togetherwith thefive-memberedrings,mayalsoberesponsible forthe signal in theintra-valley region atabout 1500cm−1.

Importantly,itiswellrecognizedthatthe rela-tiveintensityoftheDandGpeak,I(D)/I(G),can beexpressedasfunctionofthesizeofgraphitic do-mains,La[50].

Particularly, in the case of highly disor-dered/amorphous carbon materials, i.e., for very small sizeof thegraphitecrystallitessuchas the case of flame-formed soot particles [52,53], the followingempiricalexpressionhasbeenfoundto correlateLa withtherelativeintensityof thetwo Ramanpeaks[50]: L2 a nm2₌₅_.₄_{· 10}−2_{· E}4 L eV4 I(D) I(G) (1)

where EL is the energy of the incident photon. FromthemeasuredRaman spectra,we obtain a

I(D)/I(G) value of 0.76± 0.02, which results in

La=1.1nmusingEq.(1).ThisvalueofLa,roughly corresponding tothesize of ovalene, agrees rea-sonablywellwiththeaveragesizeofthearomatic buildingblocksimagedbyAFM.Asimilarsizeof thearomaticconstituentsofincipientsoothasbeen previouslyreportedusingopticalbandgapanalysis [36,56]and HR-TEM[56,57].

Furthermore, fromtheanalysisof the photo-luminescencebackgroundintheRamanspectrum, seeFig.4,wecanalsoobtainadditionalchemical information onthe nascentsootparticles. Casir-aghi et al.[58] foundanempirical equation that canbeusedfordeterminingthehydrogen to car-bonatomicratioforhydrogenatedamorphous car-bonbasedonthemeasuredratiobetweentheslope of thephotoluminescencebackgroundofthe Ra-manspectrum,m,andtheintensityoftheGband,

I(G): H[at.%]=21.7+16.6∗ log m I(G)[μm] (2)

whereH[at%]istheatomicpercentageofhydrogen fromwhichitispossibletocalculatetheH/Cratio. FromthemeasuredRamanspectra,we obtained m/I(D)=3.2± 0.5thatgives,byusingEq.(2),an averageH/Cvalueof0.43± 0.03,whichistoagood extentconsistentwiththeH/Cof thecompounds observedbyAFM.

Overall,theRamanspectroscopymeasurements agree well with the size and structure of the molecules found by STM/AFM. This indicates thatthe molecularcomposition of thesample is notalteredbythesamplepreparationprocessfor STM/AFMcharacterizationandthattheassigned PAHsareindeedbuildingblocksinearlysoot for-mation.

4. Conclusions

Incipientsootnanoparticlesgeneratedina lam-inarpremixedethylene/airflamewereinvestigated by low-temperature atomic-resolution AFMand Ramanspectroscopy.Thechemicalstructureof in-dividualaromaticbuildingblockscontributingto theformation/nucleationofsootparticlescouldbe identified, thereby confirming some previous as-sumptionsbutalsoprovidingnovel,important in-sights:

•AFM data corroborate the large complex-ity/varietyof thearomaticcompounds par-ticipatingintheformationoftheinitialsoot particles.

•Penta-ringswerefrequentlyobservedinthe imagedaromaticmolecules.

•Several molecules contained non-aromatic side-groupssuchascycloaliphaticmoieties, methylgroupsandalkylchains.

•Thereareindicationsforthepresenceof un-pairedπ–electronswithinthearomatic build-ingblocks.

•AFMdatapresentthefirstexperimental ev-idenceof cross-linkedaromaticcompounds withinthepoolofmoleculesparticipatingin thesootformation.

•The average size of aromatics moieties in incipient soot, retrieved by Raman spec-troscopy,isof theorderof 1.1nm,withan averageH/Cratioof0.43,thusingood agree-mentwiththeAFMobservations.

Acknowledgments

We thank R. Allenspach and S. Fatayer for comments anddiscussions. This workwas finan-ciallysupportedbytheERCConsolidatorGrant AMSEL(682144)andbyAccordodiProgramma CNR-MSERicercadiSistemaElettrico– Project “MIcroco/trigenerazionediBioenergiaEfficiente eStabile(Mi-Best)”.

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